The present invention relates to a semiconductor device having a smooth trench, in particular, a trench filled with a diffusion layer.
With regard to a semiconductor device having a trench filled with a diffusion layer, a manufacturing method for making a prototype of the device is shown in FIGS. 8A to 8D.
As shown in FIG. 21A, a silicon substrate 201 having a silicon oxide film 202 formed on a surface thereof is prepared. Subsequently, after an opening portion is formed in the silicon oxide film 202 at a predetermined region by photo-etching, dry etching or wet etching is performed to form a trench 203 in the silicon substrate 201 by applying the silicon oxide film having the opening portion as a mask as shown in FIG. 21B. After that, the trench 203 is filled with an epitaxial film 204 in which impurities are doped as shown in FIG. 21C. Then, the epitaxial film 204 is polished by applying the silicon oxide film 202 as a stopper, so that a polycrystalline silicon layer 205 is flattened as shown in FIG. 21D. Through the steps described above, the semiconductor device having the trench 203 filled with the diffusion layer is completed.
FIG. 22A shows an enlarged cross sectional view of the semiconductor device shown in FIG. 21B after the trench 203 is formed. FIG. 22B shows an enlarged cross sectional view of the semiconductor device shown in FIG. 21C when the epitaxial film is formed. In the method described above, the silicon oxide film 202 is used both as the mask for forming the trench and as the stopper for flattening the epitaxial film. Therefore, the epitaxial film is formed on the silicon oxide film 202 which is left as it is after the trench is formed.
However, during the step shown in FIG. 22B, the opening width of the trench 203 becomes wider than that of the opening portion of the silicon oxide film 202, so that an edge of the silicon oxide film 202 protrudes from an inner surface of the trench 203 in parallel to a surface of the silicon substrate to form a protrusion 202a as shown in FIG. 22A. Therefore, when the epitaxial film is grown in the step shown in FIG. 21C, polycrystalline silicon grows on the protrusion 202a, which causes increase of crystal defects in the epitaxial film 204. Since a growth rate of polycrystalline silicon is larger than that of single crystal silicon, an opening portion of the trench 203 is shut up by polycrystalline silicon portion 205. As a result, a void in filling the trench is generated in the trench 203.
Another problem is explained with reference to FIGS. 26A and 26B. These figures show schematic cross sectional views of a prototype device based on a cross sectional SEM image. FIG. 26A shows a schematic cross sectional view after a trench etching is performed. FIG. 26B shows a schematic cross sectional view after epitaxial growth for filling the trench is performed.
When a silicon substrate having a (110) face orientation is used as a substrate and when a trench is formed in the substrate at a width of 18 μm and a depth of 13.5 μm, an increase in the crystal defects occurs in a part of epitaxial film formed on a bottom surface of the trench. This may be caused by a roughness of Si (110) face located on the bottom surface of the trench larger than that of Si (111) face located on side face of the trench.